A main function of an automotive air intake system is to supply filtered air to the engine. In addition to that function, the air intake system is used, for example, also to supply filtered air for positive crankcase ventilation or to a secondary air charging system.
The secondary air charging system is used to supply air to an exhaust system of the engine to improve the oxidation of hydrocarbons and carbon monoxides in the exhaust gas. Each of U.S. Pat. Nos. 6,094,909 and 6,883,323 describes a secondary air charging system.
One problem associated with a current secondary air charging system is that it interferes with the operation of a mass air flow sensor, which is disposed downstream from the point at which filtered air is drawn from the air intake system to supply the secondary air charging system. The mass air flow sensor is used to measure the rate of air flow to the engine in the air intake system. Drawing filtered air from the air intake system to supply the secondary air charging system creates air turbulence in the air intake system, making it difficult for the mass air flow sensor to accurately measure air flow. It also increases the noise level in the sensor signal, making it difficult to calibrate the engine control module.
The current state of technology for overcoming this problem is to separate the air flow in the air intake system from the air flow to the secondary air charging system. To that end, a box is provided having one open side and five closed faces, and the open side of the box faces the downstream surface of the air intake system's air filter to receive filtered air. A conduit, such as a hose, is connected to the box so that filtered air can flow from the box to the rest of the secondary air charging system.
However, this arrangement has several problems. One of the problems is that the arrangement usually does not completely separate the air flow in the air intake system from the air flow to the secondary air charging system. The reason is that there is usually a gap between the open side of the box and the downstream surface of the filter medium. Although the parts can be made more precisely to reduce the gap, the gap cannot be completely eliminated due to the coarseness of the filter's downstream surface. And precisely-made parts tend to be more expensive and may be difficult to assemble. Furthermore, when the open side of the box is placed close to the downstream surface of the filter medium to reduce the gap, the open side of the box may damage the filter, causing leakage through the filter medium.
This gap allows air flow between the box and the air intake system. When the secondary air charging system is in operation, the pressure inside the box is much lower than the pressure in the air intake system. As a result, air is drawn from the air intake system to the secondary air charging system, causing turbulence in the air intake system. On the other hand, when the secondary air charging system is not in operation, the pressure in the box is higher, creating air flow from the box to the air intake system and causing turbulence in the air intake system. Furthermore, the on/off operations of the secondary air charging system cause abrupt flow changes between the box and the air intake system, also creating flow turbulence in the air intake system.